The pill claims to boost weight loss by 50 percent
The first medically approved diet pill to go on sale without a prescription hits chemists' shelves this week.
Alli works by tricking the body into passing fat straight though the digestive system instead of absorbing it.
The pills will be available for around £50 for a monthly course from most high street pharmacists on Wednesday 22nd April.
Alli's makers claim it can boost weight loss by an extra 50 per cent.
For every two kilos you lose through dieting alone; the drug can help you lose an extra kilo.
But critics are worried that customers might see it as a quick fix.
They say a sensible diet and proper exercise regime are more effective and the side effects are likely to put off all but the most dedicated users anyway.
Alli is a half strength version of the drug Xenical, which is only available with a doctor's prescription.
Like other so-called 'poo pills', it works by blocking fat digesting enzymes in the body.
That means around a quarter of the fat you eat is passed straight through the gut, sometimes with little or no warning.
On the drug's website, customers are advised to wear dark clothing and take a spare pair of trousers to work in case of 'accidents'.
Holly, 23, from Plymouth was put on the stronger prescription version of Alli a couple of years ago.
"If you passed wind, coughed or sneezed it would just be like fat coming out of your backside. It was horrible," she told Newsbeat.
"I would go to work and have to leave half way through the day. I was with my first proper boyfriend at the time as well."
But other users we spoke to were less bothered about the side effects.
Kelly from Liverpool said: "I guess it has kind of worked for me, but I don't really agree with it being sold over the counter as I don't feel that someone who works at a pharmacy is really qualified to make a decision."
"You must maintain a healthy diet combined with exercise or you will fall foul to the curse of the nasty butt!"
Dylan from Crawley reckons he lost a stone after buying a course of Alli from an American internet site, something the makers of the drug do not recommend.
"It's just an aid more than anything else. You eat your normal food and taking these tablets just brings it down to a low fat diet with the fat passing through you," he said.
"I suppose some people could see it as a quick fix. But if you're carrying a lot of weight any sort of fix is a good fix if it's going to help you out."
The Alli pill is available from chemists without prescription
The makers of Alli claim they never planned to market the pill as an alternative to a healthy lifestyle.
But critics like Professor Gareth Williams at the University of Bristol reckon putting an anti obesity drug on sale without a prescription sends out the wrong message.
"It perpetuates the myth that obesity can be fixed simply by popping a pill," he wrote in the British Medical Journal.
"It could further undermine efforts to promote healthy living, which is the only long-term escape from obesity."
Professor Williams reckons dieters could get exactly the same effect by leaving a few chips on their plate at the end of a meal, eating an apple instead of ice cream or having sex for 15 minutes.
Emma Boran from the British drugs company GlaxoSmithKline which makes Alli calls the pill a "valuable tool'' in reducing weight alongside a low-calorie diet.
She said: "We absolutely think Alli is completely suitable to be available without a prescription. You have to get it from a pharmacist and the other key thing with regard to side effects is that if you eat a low calorie, low fat diet, you won't experience those."
"Alli is for those people who are committed to losing weight. So you really have to change your perception towards food and exercise.
"If you use Alli in conjunction with that you will get an additional 50 per cent in weight loss."
The drug will be on restricted sale at most chemists and should only be available to customers over 18 and with a body mass index (BMI) of 28 or more after consultation.
Ravi Mohan from Weldricks pharmacy in Sheffield told Newsbeat: "The advantage going to pharmacy is that people can walk in and don't need an appointment.
"If I thought that this wasn't appropriate or there was any chance they were buying this for someone else then it would be a simple case of, 'sorry, we're not selling it'."